Sensor Element and Method for Producing a Sensor Element

ABSTRACT

A sensor element and a method for producing a sensor element are disclosed. In an embodiment the sensor element is configured to be secured on a printed circuit board by pressure sintering, wherein a structural form of the sensor element is designed such that an exposure to pressure of the sensor element during the pressure sintering is compensated.

This patent application is a national phase filing under section 371 of PCT/EP2016/074966, filed Oct. 18, 2016, which claims the priority of German patent application 10 2015 118 720.5, filed Nov. 2, 2015 and German patent application 10 2016 101 247.5, filed Jan. 25, 2016, each of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

A sensor element is provided. The sensor element may be used for measuring a temperature. It is, for example, an NTC sensor element (negative temperature coefficient), that is to say an NTC resistor. Methods for producing a sensor element are also provided.

BACKGROUND

According to the prior art, for monitoring and controlling temperatures in a wide variety of applications, they are mostly measured by ceramic negative temperature coefficient thermistors (NTC), silicon temperature sensors (KTY), platinum temperature sensors (PTD) or thermocouples (TC). Of these, the NTC thermistors are most commonly used, because of the low production costs. Another advantage over thermocouples and metallic resistance elements, such as, for example, Pt elements, is the significant negative resistance temperature characteristic.

For use in power modules, SMD NTC temperature sensors that are soldered on are mostly used. Also used as an alternative to this in the case of control modules for low power levels are NTC chips, which are mounted on the underside by means of Ag sintering, soldering or adhesive bonding and the upper side of which is contacted by means of a bonding wire.

For electrical contacting of the NTC ceramic, metallic electrodes have to be applied. According to the prior art, for this purpose thick-film electrodes are applied, mostly from silver or gold pastes, by means of a screen printing process with subsequent firing. The silver metallizations are particularly suitable for Ag-sintered and soldered connections. As a result of the increasing technological requirements with regard to new reliable ways of establishing electrical contact in connections, such as bonding and welding, another electrode is necessary, especially when bonding with gold or aluminum or copper wires, because a connection to silver does not have sufficient reliability.

In the case of gold metallizations, soldered connections with terminal wires cannot be realized. For reasons of cost, only thin gold wire is used for making bonded connections. Aluminum bonding wire connections on gold electrodes do not meet the reliability requirements.

At present, the temperature measurement in the case of power modules is performed by soldered-on SMD NTC sensors. As a result of the increasing requirements with respect to operating temperature and reliability, there is the requirement for NTC temperature sensors that can be applied to the mother board without soldered mounting and have high long-term stability and also are suitable for higher operating temperatures.

Suitable mounting is provided by Ag sintering with finely dispersed silver pastes, which however is performed under pressure by applying a load from above. For this purpose, sufficient mechanical stability of the components in flip-chip mounting is required, in order that the flexural stresses occurring do not lead to rupturing and there is no superficial damage (cracks). Conventional NTC sensor ceramics usually have a flexural strength that is too low to withstand the loads occurring.

SUMMARY OF THE INVENTION

Embodiments provide a sensor element that has improved properties.

According to one aspect, a sensor element for temperature measurement is provided. The sensor element may comprise a ceramic sensor material. The sensor material may be an NTC ceramic. For example, the ceramic has a perovskite structure. In particular, the ceramic may be based on the system Y—Ca—Cr—Al—O with various dopings. Alternatively, the sensor element may comprise a ceramic with a spinel structure. For example, the ceramic may be based on the system Ni—Co—Mn—O with various dopings. The sensor element may be an NTC sensor chip.

In various embodiments, the sensor element is intended to be secured on a printed circuit board or a DCB board by means of Ag pressure sintering. In particular, the sensor element may be designed to be mounted on the printed circuit board by means of Ag sintering. A structural form of the sensor element is designed such that an exposure to pressure of the sensor element during the pressure sintering is compensated.

The mechanical stability of the sensor element may be increased by the structural form according to the invention. In various embodiments, the pressure sintering of the component is thereby made possible without inducing any damage such as micro cracks or the like, or even bringing about a rupturing of the component. Consequently, a particularly stable sensor element that can be applied to the printed circuit board without soldered mounting is provided.

According to an exemplary embodiment, the sensor element has at least one electrode. Preferably, the sensor element has two electrodes. In addition, the sensor element may also have a ceramic reinforcement or a ceramic carrier. The electrodes are arranged on the ceramic sensor material.

In further embodiments, the electrodes are spatially separated from one another. This means that there is a distance between the electrodes. For example, ceramic sensor material is arranged between the electrodes. The sensor element is designed such that compressive loading occurring during the Ag pressure sintering is dissipated to the printed circuit board in an intermediate region between the electrodes. In particular, the forces acting during the Ag pressure sintering are compensated with the aid of the intermediate region between the electrodes.

Consequently, an NTC temperature sensor with electrodes suitable for Ag sintering is provided, the sensor being formed such that the compressive loading during the Ag sintering is for the most part dissipated to the mother board in the intermediate region of the electrodes and instances of damage due to flexural loading during mounting are avoided.

According to an exemplary embodiment, the sensor element has a T-shaped structural form. In particular, the sensor element has a portion that corresponds to a horizontal line or bar of a “T”. Furthermore, the sensor element has a portion that corresponds to a vertical line or bar of a “T”. The two portions are undetachably connected to one another. The two portions are preferably made in one piece.

The mechanical stability of the sensor element may be increased by the T-shaped structural form. The Ag pressure sintering of the component is thereby made possible without inducing any damage such as micro cracks or the like, or even bringing about a rupturing of the component.

According to an exemplary embodiment, the sensor element has a ceramic main body. The ceramic main body comprises the ceramic sensor material. The electrodes are arranged on an outer area of the main body. The electrodes are preferably arranged on a common outer area of the sensor element or of the main body. The common outer area preferably represents an underside of the sensor element or of the main body.

In various embodiments the main body has a projection. The projection forms an integral part of the main body. In other words, the projection and the main body are formed in one piece. In particular, the projection comprises ceramic sensor material. The projection is arranged between the electrodes. The projection forms an intermediate region between the electrodes.

The projection may be designed in the form of a base. The projection represents, for example, the vertical line of a “T”, while the rest of the main body represents the horizontal line or bar of the “T”. The projection protrudes between the electrodes out of the main area or the underside of the sensor element.

The forces occurring during the Ag pressure sintering may be dissipated to the printed circuit board through the projection or base arranged in the intermediate region between the electrodes. Damage to the sensor element due to flexural loading can consequently be prevented.

According to an exemplary embodiment, the sensor element has a ceramic main body. The ceramic main body comprises the ceramic sensor material. The electrodes are arranged on different end faces of the sensor element. The electrodes are arranged on opposite end faces. Preferably, the electrodes are applied to the end faces as caps.

The sensor element may also comprise a ceramic carrier material. The main body may be formed on the carrier material. Preferably, the main body covers an outer area of the carrier material, for example, an upper side of the carrier material, completely.

The carrier material may have a projection. The projection forms an integral part of the carrier material. In other words, the carrier material and the projection are formed in one piece. In particular, the projection comprises ceramic carrier material. The projection is arranged between the electrodes. The projection forms an intermediate region between the electrodes. The projection is designed in the form of a base. The projection projects between the electrodes out of an outer area of the sensor element.

As described above, the sensor element may have a T-shaped structural form. In particular, the sensor element has a projection or base. The projection or base protrudes from a surface of the sensor element. The projection or base is preferably formed between the electrodes. The projection or base may comprise ceramic sensor material and/or a material of the ceramic reinforcement. The structural form of the sensor element is modified or designed such that an exposure to pressure of the sensor element during the production or mounting process is compensated. Furthermore, mechanical flexural loading is reduced to a minimum.

According to one aspect, methods for producing a sensor element are described. Preferably, the sensor element described above is produced by the respective method. All of the properties that are disclosed with reference to the sensor element or the method are also correspondingly disclosed with reference to the respective other aspects, and vice versa, even if the respective property is not explicitly mentioned in the context of the respective aspect.

The method has the following steps:

Producing NTC powder to form a ceramic main body.

Pressing the NTC powder. For this purpose, a pressing mold is used. The pressing mold is designed in such a way that the pressed main body has a projection. The pressing mold is designed in such a way that the pressed main body has a T-shaped structural form.

Sintering the pressed main body.

Applying electrodes to an underside of the main body. This may be performed by means of thin-film or thick-film technology. The electrodes are separated from one another by the projection.

According to a further aspect, the method has the following steps:

Providing a ceramic carrier material. The carrier material has a projection.

At least partially printing the carrier material with an NTC paste to form an NTC layer. Preferably, a surface of the carrier material that is opposite from the base is printed with NTC paste. For example, an upper side of the carrier material is printed with NTC paste. For example, the projection is arranged on the opposite side, that is to say the underside of the carrier material. Alternatively, the projection of the carrier material may also only be formed after the printing of the carrier material with the NTC paste.

Sintering the system comprising the carrier material and the NTC paste.

Applying electrodes. The electrodes are arranged on opposite end faces of the system comprising the NTC layer and the carrier material. The electrodes are separated from one another by the projection. In order that an electrical contact is created between the mounting areas of the carrier ceramic and the NTC layer, the electrodes are designed as caps over the end faces.

According to one aspect, a sensor element for temperature measurement is provided, the sensor element having a T-shaped structural form, and the structural form of the sensor element being designed such that an exposure to pressure of the sensor element during the production process is compensated.

BRIEF DESCRIPTION OF THE DRAWINGS

The sensor element and the method are explained in more detail below on the basis of exemplary embodiments and the associated figures.

The drawings described below should not be regarded as true to scale. Rather, for better representation, individual dimensions may be shown as increased or reduced in size or even distorted.

Elements that are the same as one another or perform the same function are provided with the same designations.

FIG. 1 shows a sensor element in a first embodiment;

FIG. 2 shows a sensor element in a further embodiment;

FIG. 3 shows a sensor element in a further embodiment; and

FIG. 4 shows an embodiment of the sensor element from FIG. 3.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows a sensor element 1, in particular a sensor chip. The sensor element 1 is preferably designed for measuring a temperature. The sensor element 1 has two electrodes 2. The sensor element 1 comprises a ceramic sensor material. The sensor element 1 has a ceramic main body 7. The main body 7 comprises the ceramic sensor material.

The sensor material is an NTC (negative temperature coefficient) ceramic. For example, the ceramic has a perovskite structure. In particular, the ceramic may be based on the system Y—Ca—Cr—Al—O with various dopings. Such a sensor element 1 is particularly suitable for high-temperature applications. Alternatively, in particular in the case of lower application temperatures, the sensor element 1 may comprise a ceramic with a spinel structure. For example, the ceramic may be based on the system Ni—Co—Mn—O with various dopings.

The sensor element 1 is designed to be secured on a printed circuit board under pressure, for example, by means of Ag sintering. Ag sintering involving exposure to pressure is not possible in the case of the conventional SMD NTC sensors and alternative flip-chip structural forms because of the flexural loads occurring that exceed the intrinsic strength of the components. The flexural loading is caused by the component that lies on the terminal pads on the mother board being pressed from above. Especially in the case of DCB boards, between the terminal pads there is a relatively deep trench, which corresponds to the thickness of the electrode layer on the DCB board and is generally several 100 μm.

To compensate for the compressive loads occurring during Ag sintering, the sensor element 1 from FIG. 1 has a base 3. The sensor element 1 is designed in a T-shaped form. According to the exemplary embodiment from FIG. 1, in particular the ceramic main body 7 is designed in a T-shaped form. In this exemplary embodiment, the horizontal line of the “T” forms an upper side of the sensor element 1 or of the main body 7. The vertical line of the “T” represents the base 3. The base 3 protrudes out of an underside of the sensor element 1 or main body 7. The base 3 is an integral part of the main body 7.

The sensor element 1 with the base 3 may either be produced directly by pressing with a suitable pressing mold or be cut in a T shape from a pressed blank or a substrate produced from NTC sheets. This can be realized by sawing, grinding, laser cutting or other suitable machining operations.

The metallization is applied as a thin-film or thick-film electrode. The electrodes 2 are separated or spatially separated from one another by the base 3.

The electrodes 2 are applied on the underside to the right and left of the base 3, in particular by means of sputtering, vapor deposition or screen printing. The electrodes 2 are arranged on the same outer area (here the underside) of the sensor element 1.

To achieve good adhesive attachment in the Ag pressure sintering process, an electrode that likewise consists of Ag is of advantage. However, other electrode materials, such as, for example, Au, Cu, Al, etc., may also be used, as long as they are Ag-sinterable or can be processed by some other standard process.

The production of thin-film electrodes may be performed by sputtering or vapor deposition. In this case, in a first embodiment the base electrode consists of a nickel layer, which may comprise fractions of vanadium, or in a second embodiment of two layers, the lower layer comprising chromium or titanium and the second layer consisting of nickel, which likewise may comprise fractions of vanadium.

The base electrode may be protected by a covering layer consisting of an oxidation-inhibiting metal such as, for example, silver, gold, copper, aluminum, etc. This covering electrode may either just serve for protecting the nickel base electrode from corrosion (oxidation) or else be advantageous or even necessary for contacting. In the case of a connection by means of Ag sintering with finely dispersed silver pastes, for example, a silver covering electrode is indispensable.

The thickness of the base electrode is less than 10 μm, advantageously less than 3 μm, ideally less than 0.5 μm. The thickness of the covering electrode may be up to 1 μm, in exceptional cases up to 20 μm.

The production of thick-film electrodes may be performed by a screen printing process with subsequent firing. The pastes used may contain Ag or any admixtures.

The final geometry is produced by a cutting process. In the case of very closely toleranced resistances, a trimming process may be performed for setting the resistance at nominal temperature by partial laser ablation. The contacting of the sensor element 1 with respect to the DCB board or the printed circuit board or the mother board may be performed by means of Ag sintering, soldering or adhesive bonding.

According to this exemplary embodiment, the sensor element 1 (pressed blank) is produced, for example, in the following way:

In a first step, NTC powder is produced. This comprises, for example, initial weighing, wet pre-grinding, drying, screening, calcining, wet after-grinding and spraying. After that, the pressing of the granular sprayed material is performed. The pressing mold is in this case designed such that a T-shaped main body is created during the pressing.

The decarburizing of the pressed blank follows in a further step. After that, the pressed blank is sintered.

The application of Ni/Ag thin-film electrodes 2 to the undersides to the right and left of the base 3 is performed by means of sputtering technology, as described above. The electrodes 2 are separated from one another by the base 3.

To improve the long-term stability of the ceramic, in a further step a thin, nonconducting protective layer, which consists, for example, of ceramics, glasses, plastics or metal oxides, may be applied over the unmetallized region. This can be achieved by sputtering, vapor deposition, lithography or printing and firing.

After that, the electrical measuring of the resistances of the individual sensor elements 1 with base 3 at nominal temperature is performed. For setting the resistance, the metalized substrates are electrically measured in advance. The geometry of the NTC sensor chips with base is defined on the basis of the measurement data obtained in advance. Since the length is fixed, the width remains as a variable setting parameter.

In a further step, a trimming of the individual sensor element 1 with base 3 to the required resistance value is performed by grinding the full surface area of one side.

For particularly closely toleranced resistances at nominal temperature, the resistance of the individual components can be set by the additional trimming process (also see in this respect FIG. 4). In this case, ceramic material or electrode material is partially removed, for example, by laser cutting or grinding, in such a way that the resistance is adapted by changing the geometry.

A visual inspection and random control measurement follow in a final step.

FIG. 2 shows a reinforced sensor element 1 with base 3. The construction corresponds substantially to the sensor element 1 from FIG. 1. However, the sensor element 1 has an additional ceramic layer 4 on the upper side (the side opposite from the base 3). The main body 7 is arranged on the ceramic layer 4. In particular, the ceramic layer 4 covers the upper side of the main body 7 preferably completely. The ceramic layer 4 serves as mechanical reinforcement for processes with particularly high stresses. The construction is realized by pressing granular material or stacked sheets. The NTC ceramic is pressed either together with the ceramic layer 4 or one after the other. The production of the sensor element 1 according to FIG. 2 is otherwise performed as described in connection with FIG. 1.

FIG. 3 shows a carrier material 5 with base 6 printed with sensor material (NTC layers). In this exemplary embodiment, the NTC layers represent the main body 7.

The NTC layers are printed onto the ceramic carrier material 5. The carrier material 5 is designed in a T-shaped form. In particular, the carrier material 5 has the base 6, the base representing the vertical line of the “T”. The base 6 is an integral part of the carrier material 5.

The ceramic carrier material 5 consists on the basis of, for example, Al₂O₃, ZrO₂, ATZ or ZTA materials or MgO. The carrier material 5 may be brought into an appropriate form before or after being printed with NTC paste and individually separated after the sintering, or already take the form of a single part.

The electrodes 2 are applied to the end faces or side faces. In particular, the electrodes 2 are applied as caps. This allows the contacting of this structural form on the printed circuit board or mother board or the DCB board. Since the sensor material (the NTC layer) is not however in direct contact with the pads, the cap form of the electrodes 2 is required to ensure contacting of the NTC layer.

FIG. 4 shows a trimmed NTC sensor on a carrier material 5 with base 6. Sensor material has been removed by the trimming process for setting the resistance at nominal temperature. The removal is performed by partial laser ablation, as already described above.

According to all of the exemplary embodiments shown in FIGS. 1 to 4, the sensor element 1 has a T-shaped structural form. The high forces acting on the component during the pressure sintering are compensated by the structural form chosen, and the mechanical flexural loading is reduced to a minimum. In particular, the compressive loading during the Ag sintering is for the most part dissipated to the mother board in the intermediate region of the electrodes 2, whereby instances of damage due to flexural loading during mounting are avoided.

The use of ceramic carrier materials on the basis of, for example, Al₂O₃, ZrO₂, ATZ or ZTA materials or MgO can lead to a further increase in the mechanical stability.

For use on mother boards or DCB boards, the sensor elements 1 shown in FIGS. 1 to 4 may be sintered onto the conductor tracks. This may be performed under pressure or without pressure. Mounting by adhesive bonding or soldering continues to be applicable.

The description of the subjects specified here is not restricted to the individual specific embodiments. Rather, the features of the individual embodiments can—as far as technically feasible—be combined with one another in any desired manner. 

1-14. (canceled)
 15. A sensor element for temperature measurement, wherein the sensor element is configured to be secured on a printed circuit board by pressure sintering, and wherein a structural form of the sensor element is designed such that an exposure to pressure of the sensor element during the pressure sintering is compensated.
 16. The sensor element according to claim 15, wherein the sensor element has at least two electrodes, and wherein the sensor element is designed such that a compressive loading occurring during the pressure sintering is dissipated to the printed circuit board in an intermediate region between the electrodes.
 17. The sensor element according to claim 16, wherein the sensor element has a ceramic main body, wherein the electrodes are arranged on an outer area of the main body, wherein the main body has a projection, and wherein the projection is arranged between the electrodes.
 18. The sensor element according to claim 17, wherein the electrodes is arranged on a common outer area of the sensor element, and wherein the common outer area represents an underside of the sensor element.
 19. The sensor element according to claim 17, wherein the projection is designed as a base, and wherein the projection protrudes between the electrodes out of the outer area of the sensor element.
 20. The sensor element according to claim 17, wherein the projection forms an integral part of the main body.
 21. The sensor element according to claim 16, wherein the sensor element has a ceramic main body and comprises a ceramic carrier material, wherein the main body is formed on the carrier material, wherein the carrier material has a projection, and wherein the projection is arranged between the electrodes.
 22. The sensor element according to claim 21, wherein the electrodes are arranged on different end faces of the sensor element.
 23. The sensor element according to claim 22, wherein the electrodes are caps of the end faces.
 24. The sensor element according to claim 21, wherein the projection is designed as a base, and wherein the projection projects between the electrodes out of an outer area of the sensor element.
 25. The sensor element according to claim 21, wherein the projection forms an integral part of the carrier material.
 26. The sensor element according to claim 15, wherein the sensor element has a T-shaped structural form.
 27. A method for producing a sensor element, the method comprising: producing NTC powder to form a ceramic main body; pressing the NTC powder using a pressing mold, wherein the pressing mold is designed in such a way that the pressed main body has a projection; sintering the pressed main body; and applying electrodes to an underside of the main body, the electrodes being separated from one another by the projection.
 28. A method for producing a sensor element, the method comprising: providing a ceramic carrier material, the carrier material having a projection; at least partially printing the carrier material with an NTC paste to form an NTC layer; and applying electrodes on opposite end faces of the system comprising the NTC layer and the carrier material, the electrodes being separated from one another by the projection. 